Balls for use in a bearing (hereinafter called bearing balls) are generally made of metal, such as bearing steel. However, in view of higher wear resistance, bearing balls made of ceramic are becoming popular. In this connection, an effort to enhance quality of ceramic balls has been earnestly pursued.
A measure for enhancing quality of a ceramic ball is to reduce pores present in the ceramic ball. Pores present in a ceramic ball cause a failure to obtain a required strength, a failure to attain good dimensional accuracy even through surface polishing, or increase the surface roughness of the ceramic ball. Thus, in order to reduce pores in a ceramic ball, various devices are employed. An important point is to increase the relative density of a green body to the greatest possible extent.
A green body of high density can be obtained through employment of a wet, cold isostatic pressing (CIP) process. Specifically, as shown in FIG. 3, preliminary green bodies 104′, each formed into a spherical shape through preliminary formation by, for example, a die pressing process, are contained in a rubber die 106. Then, the loaded rubber die 106 is placed in a soft, flexible resin bag (e.g., a polyurethane bag) 105 in a sealed condition, followed by vacuum packing. Subsequently, pressure is applied to the sealed bag 105 in a substantially isostatic condition through application of static pressure by means of a liquid pressure-application medium, such as oil or water. However, since the wet CIP process involves placement of preliminary green bodies in a resin bag in a sealed condition, vacuum packing, and removal of formed green bodies from the resin bag, the process is very poor in work efficiency, unavoidably resulting in increased manufacturing cost. In order to cope with the problem, in a dry CIP process that is becoming popular, a liquid pressure is indirectly applied, via an outer rubber die, to an inner rubber die loaded with preliminary green bodies.
However, the conventional CIP process involves difficulty in attaining uniform densification of a spherical green body. As a result, pores tend to remain in a sintered body. This problem tends to arise particularly in dry CIP, in which a liquid pressure is indirectly applied to a green body via an outer rubber die.
An object of the present invention is to provide an efficient, low-cost method for manufacturing a ceramic ball having few pores and uniform density distribution, a ceramic ball manufactured by the method, a ball bearing using the ceramic ball, a motor having a bearing using the ceramic ball, a hard disk drive using the ball bearing, and a polygon scanner using the ball bearing.